title: Milrinone attenuates thromboxane receptor-mediated hyperresponsiveness in hypoxic pulmonary arterial myocytes.
abstract: OBJECTIVE
  Neonatal pulmonary hypertension (PPHN) is characterized by pulmonary vasoconstriction, due in part to dysregulation of the thromboxane prostanoid (TP) receptor. Hypoxia induces TP receptor-mediated hyperresponsiveness, whereas serine phosphorylation mediates desensitization of TP receptors. We hypothesized that prostacyclin (IP) receptor activity induces TP receptor phosphorylation and decreases ligand affinity; that TP receptor sensitization in hypoxic myocytes is due to IP receptor inactivation; and that this would be reversible by the cAMP-specific phosphodiesterase inhibitor milrinone.
  
  
  METHODS
  We examined functional regulation of TP receptors by serine phosphorylation and effects of IP receptor stimulation and protein kinase A (PKA) activity on TP receptor sensitivity in myocytes from neonatal porcine resistance pulmonary arteries after 72 h hypoxia in vitro. Ca(2+) response curves to U46619 (TP receptor agonist) were determined in hypoxic and normoxic myocytes incubated with or without iloprost (IP receptor agonist), forskolin (adenylyl cyclase activator), H8 (PKA inhibitor) or milrinone. TP and IP receptor saturation binding kinetics were measured in presence of iloprost or 8-bromo-cAMP.
  
  
  RESULTS
  Ligand affinity for TP receptors was normalized in vitro by IP receptor signalling intermediates. However, IP receptor affinity was compromised in hypoxic myocytes, decreasing cAMP production. Milrinone normalized TP receptor sensitivity in hypoxic myocytes by restoring PKA-mediated regulatory TP receptor phosphorylation.
  
  
  CONCLUSIONS
  TP receptor sensitivity and EC(50) for TP receptor agonists was regulated by PKA, as TP receptor serine phosphorylation by PKA down-regulated Ca(2+) mobilization. Hypoxia decreased IP receptor activity and cAMP generation, inducing TP receptor hyperresponsiveness, which was reversed by milrinone.
authors.affiliation: Departments of PhysiologyPediatrics, University of Manitoba, Winnipeg, Canada.
authors.forename: K T
authors.initials: KT
authors.lastname: Santhosh
authors.forename: O
authors.initials: O
authors.lastname: Elkhateeb
authors.forename: N
authors.initials: N
authors.lastname: Nolette
authors.forename: O
authors.initials: O
authors.lastname: Outbih
authors.forename: A J
authors.initials: AJ
authors.lastname: Halayko
authors.forename: S
authors.initials: S
authors.lastname: Dakshinamurti
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism; D016285:Iloprost / Q000494:pharmacology; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism; D020105:Milrinone / Q000494:pharmacology*; D032389:Myocytes, Smooth Muscle / Q000166:cytology* / Q000187:drug effects*; D010100:Oxygen / Q000494:pharmacology*; D011651:Pulmonary Artery / Q000166:cytology*; D017482:Receptors, Thromboxane / Q000819:agonists / Q000037:antagonists & inhibitors / Q000378:metabolism*; D015398:Signal Transduction; D013552:Swine; D014665:Vasodilator Agents / Q000494:pharmacology*
chemical_list: D017482:Receptors, Thromboxane; D014665:Vasodilator Agents; D015544:Inositol 1,4,5-Trisphosphate; D017868:Cyclic AMP-Dependent Protein Kinases; D016285:Iloprost; D020105:Milrinone; D010100:Oxygen; D002118:Calcium